1. Field of the Invention
This invention relates to a printing sheet which is adapted for forming dye images thereon by heating an ink ribbon according to image information by use of a thermal head or a laser beam to melt or diffuse the dye in an imagewise pattern and receiving the dye image on the printing sheet.
2. Description of the Prior Art
As is known in the art, there have been widely used thermal transfer recording methods wherein an ink ribbon is heated according to image information by use of a thermal head or a laser beam to thermally melt or diffuse the ink from the ink ribbon and the thus melted or diffused dye is transferred on a printing sheet. In recent years, attention has been paid to so-called sublimation-type thermal transfer recording methods wherein full color images with a continuous tone or gradation are formed using thermally diffusable dyes such as sublimable dyes. For instance, attempts have been made to form images on a video printing sheet by spottedly heating an ink ribbon according to signals of video images.
More particularly, as shown in FIG. 1, there is used a video printing sheet 1 which includes a sheet substrate 2 such as of polypropylene and a dye image-receiving layer 3 formed on the substrate 2. The dye image-receiving layer 3 is able to receive a dye transferred from an ink ribbon by heating and keeps the resultant image thereon. The image-receiving layer 3 has been conventionally made of resins which are susceptible to dyeing with dyes. Such resins include thermoplastic resins such as polyesters, polycarbonates, polyvinyl chloride, vinyl chloride copolymers such as vinyl chloride-vinyl acetate copolymers, polyurethanes, polystyrene, AS resins, ABS resins and the like.
Recently, in order to enhance sensitivity enough to form clear images and to improve the weatherability, light fastness and thermal stability of images so that once formed images can be stably kept, various attempts have been made on resins for the dye image-receiving layer. For instance, in order to improve the light fastness and weatherability of images, there has been proposed a dye image-receiving layer which is made mainly of cellulose esters.
However, as the thermal transfer recording methods have been widespread, there is an increasing demand for solving the following problem in practical applications, not to mention the improvements in the weatherability, light fastness and thermal stability. More particularly, the image formed on known printing sheets is not resistant to sebum. When the image surface is rubbed with the hand, the dye may be attached to the hand in some case. Thus, a problem to solve is to the improvement of the printing sheet in sebum resistance. Moreover, there is some demand where letters or characters are written directly on the printing sheet by use of oil base ink pens. The ink of the pen is repelled with the known printing sheets, thus not satisfying the demand. Thus, another problem involved in known printing sheets is how to improve the writing properties of printing sheet so as to permit direct writing on the sheet by means of oil base ink pens.
The reason why the dye is attached to the hand on rubbing of the image surface therewith is considered as follows: there occur at the same time extraction of the dye with the sebaceous matter from the hand and mechanical separation of the dye from the dye image-receiving layer on rubbing of the image surface with the hand or finger. In order to improve the sebum resistance to an extent that no dye is attached to the hand on rubbing of the image surface, it is desirable to use resins for the dye image-receiving layer which have high oil repellence, the capability of securely fixing dyes on the layer and good film-forming properties.
On the other hand, in order to improve writing properties by which direct writing on printing sheet with oil base ink pens is possible, it is necessary that dyes or inks dispersing dyes therein be well infiltrated into the dye image-receiving layer. The resins for such image-receiving layer are contrary to the case of improving the sebum resistance and should be oleophilic in nature, not oil-repellent.
As will be apparent from the above, resins for the image-receiving layer should have both properties required to improve the sebum resistance and properties required to improve writing properties, which are contrary to each other in nature. To improve both properties is difficult. For instance, the improvement of the sebum resistance by crosslinkage of resins through isocyanates results in a sacrifice of writing properties.